PROJECT ABSTRACT Small GTPases and evolutionarily-related heterotrimeric G proteins cycle between GDP-bound forms that are typically considered ?off? and GTP-bound forms that directly engage downstream effectors to control diverse cellular processes. Mutations in these proteins often disrupt this nucleotide cycling, and in particular, mutations that prevent the intrinsic hydrolysis of bound GTP lead to constitutively active GTPases that contribute to a variety of human diseases - most notably cancer. Despite the significance of constitutively active GTPases in promoting human diseases, it has been difficult to target these proteins using conventional small molecule inhibitors. We propose to integrate several developing technologies to potently and selectively target constitutively active GTPases in cancers. These technologies include the use of directed evolution by mRNA display to select peptides that bind with high affinity and specificity to active GTPases; advanced synthetic chemistries to convert selected peptides into bioavailable peptidomimetics; and several new targeted delivery systems, including nanoparticles and ligand-peptide conjugates, to deliver these peptides and peptidomimetics to tumors for efficient dispersal and GTPase inhibition. In addition, these peptides are being used to enable new research directions including: i) unique high-throughput screens to reassess the potential to identify small molecule inhibitors of active GTPases from conventional, drug-like libraries and ii) the creation of biosensors to monitor the activation of GTPases with high spatiotemporal resolution.